1. Preheat grill to medium-high heat.
2. Combine 1 cup water, 1 tablespoon salt, and sugar in a shallow dish. Add fillets; let stand 15 minutes. Remove fillets; pat dry. Discard brine. Place fillets, skin side down, on a grill rack coated with cooking spray. Grill 10 minutes or until desired degree of doneness. (Do not turn fillets.) Keep warm.
3. Combine parsley leaves, onion slices, orange rind, orange sections, and tarragon in a bowl. Sprinkle herb mixture with 1/4 teaspoon salt; toss. Combine walnuts and remaining ingredients, stirring with a whisk until blended. Stir in remaining 1/4 teaspoon salt. Place 1 fillet on each of 4 plates; top each serving with 1 cup herb mixture. Drizzle 2 tablespoons walnut mixture over each serving.